1. Field of the Invention
The present invention relates to a water repellent silica sol and a process for preparing such a sol.
2. Description of Prior Art
Water repellent silica is known as a reinforcer for elastomers, plastics, waxes, etc., or as a thickening agent or viscosity modifier for liquid resins, paints and the like. The conventional water repellent silica powders, however, were incapable of being homogeneously dispersed in non-polar organic solvents such as toluene, and it was necessary to body the silica solution when mixing it with a resin or other material. Thus, there has been increasing call for a water repellent silica which can be dispersed homogeneously in non-polar organic solvents.
Various methods have been proposed for the production of water repellent silica, and these methods can be roughly classified into dry processes and wet processes.
Among the dry processes is, for example, a method which comprises reacting silica with vapor of an organosilicon halide compound at a high temperature (U.S. Pat. No. 2,801,185). This method requires a treatment for perfectly removing water from hygroscopic silica, and therefore it is necessary to perform a complicated treatment such as high-temperature treatment, vacuum deaeration treatment, etc., for effecting perfect removal of water from silica. Also, silica obtained according to this method is large in particle size and has difficulty in being dispersed in non-polar organic solvents.
The wet processes include a method which comprises mixing an alkyltrialkoxysilane with water and if necessary an acid, and adding the mixture to a silica aquasol to perform preliminary hydrolysis of the alkyltrialkoxysilane and subsequent reaction of the preliminarily hydrolyzed alkyltrialkoxysilane with the silica aquasol (U.S. Pat. No. 4,177,315). According to this method, since the reaction is carried out in the presence of a large quantity of water, the alkyltrialkoxysilane used as a reactant is liable to undergo self-condensation therebetween rather than to react with the silica particle surfaces, so that the loss of the reactant is too great and the desired water repellency can not be obtained. Further, the obtained silica particles are large in size and hard to disperse in non-polar organic solvents.
An another wet process, there is known a method which comprises reacting silica with an alkylhalosilane in an alcohol (Japanese Patent Publication No. 1090/1990). In this method, since the reactant alkylhalosilane is reacted not only with silica but also with an alcohol, the loss of the reactant is great and the desired water repellency is hardly obtainable. Also, the obtained silica particles are large in size--around 200 nm--and hard to disperse in non-polar organic solvents.